We propose to continue research in the general area of spontaneous and induced variatton and mutation in somatic cells in culture. The cells used, especially hamster fibroblasts, human lymphoblasts, and human fibroblasts, range from frankly heteroploid to truly diploid. Our objctives are: 1. Dihydrofolate reductase-specific messenger RNA: Investigate further the molecular nature of the unique variants in heteroploid hamster (BHK) lines which overproduce dihydrofolate reductase, concentrating on the isolation of specific mRNA; 2. Reductase-deficient variant: Isolate BHK variants deficient in dihydrofolate reductase; 3. Transcription and replication in synchronized human lymphoblasts: Pursue the possibilities of orderly transcription and of specific mutagenesis in synchronized cultures of near-diploid lymphoblasts; 4. Reversible non-cycling state in lymphoblasts: Confirm and study the occurrence, production and characteristics of non-cycling (GO) cells in lymphoblast cultures; 5. Variation and mutation in near-diploid lymphoblasts: Isolate certain mutants in near-diploid lymphoblasts, e.g., heterozygous at an autosomal drug-resistant locus or homozygous at an autosomal auxotrophic locus; 6. Mutagenesis in diploid lymphoblasts: Demonstrate in newly-initiated diploid lymphoblast cultures chemical mutagenesis to hypoxanthine- guanine phosphoribosyltransferase (HGPRT)-deficiency; 7. Heteroploidy in senescence and neoplasia: Determine with premature chromosome condensation the exact frequency of heteroploidy in senescent human fibroblasts, and perform certain experiments involving exposures to both oncogenic viruses and mutagens which relate to the role of heteroploidy in neoplasia. Bibliographic references: Chang, S.E. and Littlefield, J.W.: Regulation of dihydrofolate reductase in methotrexate-resistant baby hamster kidney cells. Fed. Proc., 34: 706, 1975. Kamely, D. and Littlefield, J.W.: Mutagenesis to norleucine-resistance in baby hamster kidney cells. Exp. Cell Res., 89: 154-160, 1974.